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MathGroup Archive 2002

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Re: How Do You Reduce Multiple Elements in a List?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33879] Re: How Do You Reduce Multiple Elements in a List?
  • From: Tim Brophy <timbrophy at eircom.net>
  • Date: Sun, 21 Apr 2002 06:12:29 -0400 (EDT)
  • References: <a9r3ve$ltc$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Bob,

    This is a problem given at the 1990 programming competition at the
Mathematica conference. The following code, taken from Ilan Vardi's book
"Computational Recreations in Mathematica" will work:

In[1]:=Attributes[times]={Flat}

General::spell1: Possible spelling error: new symbol name "times" is similar
to existing symbol "Times".

Out[1] = {Flat}
 
In[2]:= times[{a_,m_},{a_,n_}]:=times[{a,m + n}]

In[3]:= RunEncode[x_List]:= List @@ times @@ ({#,1}& /@ x)

In[4]:= RunEncode[{1,1,1,1,1,2,2,2,4,4,4,4,4,4}]

Out[4] = {{1,5},{2,3},{4,6}}


On 4/20/02 9:05 AM, in article a9r3ve$ltc$1 at smc.vnet.net, "Bob Harris"
<nitlion at mindspring.com> wrote:

> Howdy,
> 
> This seems like such a simple thing, yet I've search through the big book
> trying to figure it out.  Perhaps someone can help me...
> 
> Given a list of integers that generally contains the same number many times,
> I'd like to reduce it to a list that just gives me the number/count pairs.
> For example, I'd like to convert the list
>  {1, 1, 1, 1, 1, 2, 2, 2, 4, 4, 4, 4, 4, 4}
> into
>  {{1,5}, {2,3}, (4,6}}
> 
> Is there a built-in function to do this?  Seems like a histogramming
> operation, but I get no useful matches from ?*Hist*.
> 
> Thanks,
> Bob H
> 
> 


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